The present invention relates in general to superconducting circuits, and in particular to an apparatus for shielding a superconducting integrated circuit chip from external magnetic fields.
The logic gates that use the Josephson devices operate with an extremely fast speed and are expected to play a key role in the ultra-fast computers and processors of the future. Particularly, and for this reason, there are intensive efforts made to fabricate the superconducting integrated circuits that use Josephson devices wherein niobium (Nb) or its alloy is used for the Josephson junction.
Generally, the Josephson devices are extremely sensitive to magnetic fields and are used for extremely high-sensitive magnetometers capable of detecting a magnetic field of 10.sup.-10 times smaller than the earth's magnetic field.
On the other hand, the ultra-high sensitivity of the Josephson devices to magnetic fields raises a problem when one uses the Josephson devices for constructing a computer system. The operation of the Josephson devices, used in the form of superconducting integrated circuits, would be influenced when there is an external magnetic flux trapped in the Josephson devices. More specifically, such a trapped magnetic flux is pinned in the Josephson devices and modifies the operational characteristics thereof significantly. When this occurs, the switching operation of the logic devices is influenced unwantedly, and the processor would be caused to malfunction. Unless the trapping of the magnetic flux is eliminated, the Josephson processor continues operating erroneously.
Because of the foregoing reason, the superconducting integrated circuit chips are generally placed in a magnetic shield enclosure such that no external magnetic fields can penetrate into the Josephson devices.
FIG. 1 shows a conventional magnetic shield enclosure used for this purpose.
Referring to FIG. 1, liquid helium 4 is filled in a Dewar vessel 1 in which first and second magnetic shield vessels 2 and 3 are immersed to form a double wall enclosure against the external magnetic fields. Further, in the space formed within the inner vessel 3, a superconducting integrated circuit chip 5 is accommodated in contact with the liquid helium 4.
The vessels 2 and 3 are formed from a material such as permalloy (registered trademark of Western Electric, Inc.) which exhibits a high permeability in the low temperature environment. The construction of FIG. 1 may have a cap of permalloy (not shown) to close the top of the vessels 2 and 3. In this construction, the external magnetic fluxes are collected by the vessels 2 and 3 and the space within the vessel 3, in which the superconducting integrated circuit chip 5 is accommodated, becomes substantially free from magnetic flux.
FIG. 2 shows another conventional construction of a magnetic shield enclosure, wherein there is provided a heating fixture 6 in the space of the vessel 3 adjacent to the superconducting integrated circuit chip 5 to heat the chip 5 to the normal conduction state before operating the same in the superconduction state. The heating fixture 6 is configured to induce a temperature gradient within the chip 5 such that, upon cooling to the superconduction state after the foregoing transition to the normal conduction state, the superconduction state appears at a location inside the chip 5 and develops laterally along the chip 5. Thereby, the region of the superconduction state expels the magnetic flux by the Meissner effect and the magnetic fluxes that interlink the chip 5 are expelled from the chip 5.
In either the construction of FIG. 1 or FIG. 2, satisfactory magnetic shielding is not obtained when a single vessel is employed. Thus, in order to achieve the desired magnetic shielding, it is necessary to increase the number of vessels. However, the vessel itself creates a small magnetic flux, and because of this, the magnetic flux penetrating the space around the chip 5 cannot be reduced below 10.sup.-4 of the earth's magnetic field. Even in the case of achieving the foregoing reduction of the magnetic flux, which is not satisfactory, one needs to use utmost care in the manufacturing of the vessels 2 and 3 with respect to the form thereof and internal strain therein such that the efficiency of magnetic shielding is maximized. When there is a remaining magnetic flux produced by the vessel itself, such a magnetic flux may be trapped again upon cooling of the chip 5 made after the transition to the normal conduction state. Thereby, the problem of the undesirable modification of the operational characteristic of the Josephson devices remains uneliminated. Further, the heating of the chip 5 by the heating fixture 6 inevitably causes a loss of the liquid helium. When the Josephson devices are subject to the influence of external magnetic fields as such, the logic processors utilizing the Josephson devices inevitably cause malfunctioning which is detrimental to the computer system.